Gowing Potatoes

can anyone explain why my potatoe (stem/leaves )grow up to about 4feet tall i have tried putting the tubers in deep and i also earth up, i have to somehow put a tie round to stop them falling over, i have tried them in 2or 3 different locations in the garden but get the same result, i do not get many potatoes at the end, My soil is nice and easy to dig,